1. Field of the Invention
The present invention concerns spark erosion electrode wires, which are used to machine electrically conductive material parts in a spark erosion machine.
2. Description of the Prior Art
The spark erosion electrode wire generally comprises a core or at least one intermediate layer of a metal that is a good conductor of electricity, and a surface layer of the same or a different metal or alloy. The electrode wire is moved axially in a sparking area, in which it is guided by entry guides and exit guides. An electrical voltage is applied between the electrode wire and the part to be machined which is itself placed in the sparking area, in order to produce in said sparking area streams of sparks between the wire and the part. The sparks simultaneously wear away the wire and wear away the part. A dielectric liquid occupies the sparking area between the wire and the part. In some spark erosion machines, the liquid is a bath, which is circulated to evacuate waste. In other machines, the liquid is sprayed.
A surface of the required shape is cut into the part by relative displacement of the wire and the part in the radial direction relative to the axis of the wire.
The electrode wires used generally have a small diameter, frequently around 0.2 millimeters, and they must be correctly tensioned in the sparking area to achieve a good accuracy of machining.
A first problem encountered with electrode wire spark erosion machines is the frequent risk of breakage of the wire. Should the wire break, the spark erosion machine is generally adapted for the wire to be rethreaded either automatically or manually. This obviously reduces the efficiency of the machine, and threading of the wire can sometimes be very difficult because of its small diameter, its flexibility, and the small diameter of the wire guides in which it threads.
It is found that the risk of breakage tends to increase as the machining of the parts progresses.
A second problem encountered in spark erosion using electrode wires is that of obtaining a satisfactory surface state. Indeed, spark erosion operates by sparking, that is to say by the erosive action of discontinuous electrical sparks on the part. The discontinuous nature of the sparks produces a surface state on the part that is not always sufficiently regular, depending on the requirements and the applications of the part. To obtain sufficient dimensional accuracy of the machined surface, the machines carry out the machining in a number of steps, with a final finishing step in which very little material is removed from the part. Nevertheless, the roughness coefficient usually obtained on the surface of the part is not always sufficient, and visible striations are observed on the surface of the part. The existing solutions to improve the surface state obtained entail reducing the intensity of the sparking current, and/or reducing the sparking time. However, this significantly reduces the machining rate. In all cases, there is a requirement to improve the surface state and the dimensional precision of the machined parts, and to avoid their deterioration during machining.
Some electrode wires, such as brass wires with a surface layer of zinc, are known to procure a better surface state, compared to a wire with a surface layer of diffused zinc and copper alloy, but machining is then slower and the risks of the wire breaking are increased.
Various electrode wire structures have been proposed, including a surface layer of a transition metal or a metal oxide: in EP-A-0 381 595, there is a surface layer of iron, nickel, cobalt or chromium; in JP-A-01 078724, the surface layer is of manganese; in JP-A-61 279433 and JP-A-61 288932, the surface layer is of chromium oxide; in JP-A-62 028123, the surface layer is of molybdenum oxide or vanadium oxide. None of these structures improves the surface state of the machined part in combination with reducing the risk of the wire breaking and increasing the machining rate.
A problem addressed by the invention is that of designing a new spark erosion electrode wire structure reducing the risk of the wire breaking and improving the surface state of the part machined with said wire, without reducing the machining rate.